Packet data latency is one of the performance metrics that vendors, operators, and end-users (via speed test applications) regularly measure. Latency measurements are done in all phases of a radio access network system lifetime: when verifying a new software release or system component, when deploying a system, when the system is in commercial operation, etc.
Better latency than previous generations of Third Generation Partnership Project (3GPP) Radio Access Technologies (RATs) is one performance metric that guided the design of Long-Term Evolution (LTE). LTE is now recognized by the end-users to be a system that provides faster access to internet and lower data latencies than previous generations of mobile radio technologies. However, since the introduction of LTE, several improvements have been introduced to increase data rates of the system: Carrier Aggregation, Multiple-Input-Multiple-Output (MIMO), etc. To get the full benefit of these data rate enhancements, continuous enhancements of the latency of LTE are needed in the future evolution track of LTE towards 5G.
Radio resource efficiency could be positively impacted by latency reductions. Lower packet data latency could increase the number of transmissions possible within a certain delay bound; hence higher Block Error Rates (BLER) targets could be used for the data transmissions, freeing up radio resources potentially improving the capacity of the system.
Therefore, one area to address when it comes to packet latency reduction is the reduction of transport time of data and control signaling by addressing the length of a Transmission Time Interval (TTI) and the reduction of processing time of control signaling (e.g. the time it takes for a User Equipment (UE) to process a grant signal).
One possible solution is to shorten the TTIs by introducing a short TTI (sTTI) concept. The shorter TTIs can have any time duration. As one example, the duration of the sTTI may be 0.5 ms. It is assumed that the UE and network processing related to e.g. Hybrid Automatic Repeat Request (HARQ) retransmissions can be reduced compared to the legacy TTI operation.
In the legacy operation, the HARQ feedback for a Physical Downlink Shared Channel (PDSCH) transmission in subframe n occurs in subframe n+4. For simplification, it can be assumed that the HARQ feedback delay for a short PDSCH (sPDSCH) can be scaled with the same factor as the sTTI length compared to the 1 ms TTI used today.